These studies are aimed at understanding the developmental pathways and functional heterogeneity of different mouse B lymphocyte subpopulations. Lymphocytes are being classified according to surface phenotype using monoclonal antibodies and fluorescence-activated cell sorter analysis, and according to function using antigen-dependent B cell activation in an in vitro serum-free defined culture system. In addition, cloned T cell lines are being used to activate B-cell immunoglobulin synthesis in order to determine the number of receptors on B cells for T lymphocyte helper factors.